The present disclosure relates to interconnect structures used in packaged microelectronic assemblies. In particular it relates to interconnect structures used in the connection of microelectronic chips and dies to substrates, such as in flip-chip bonding, or between substrates, such as in the formation of stacked packages. The structures described herein can be used to reduce failure of interconnect bonds due to void formation caused by electromigration between components of prior structures.
Electromigration is a main cause of interconnect failure, especially in high-performance devices where the current density in the interconnect and the device operating temperatures are high. In general, electromigration is caused by the different diffusion rates of materials used in interconnect assemblies. For example, an interconnect assembly can include a contact pad made of copper formed on each of two substrates and a solder mass bonded between the contact pads. The solder mechanically secures the two contact pads, and thus, the substrates on which they are formed, and also electronically connects the two pads so that a signal carried by an electric current can pass between the two pads through the solder mass. In this example, the diffusion rates between the solder and the copper of the pads can be different. The diffusion rates are the rates of molecular movement within the metallic structures over time, and in particular, when subjected to a current or to the heat caused by operation of the devices.
Voids formed in interconnect structures can decrease the reliability of the microelectronic assemblies in which they are used. Further, the presence of voids increases the current density within the materials in the areas surrounding the voids. This can, in turn, further exacerbate the difference in diffusion rate, leading to acceleration of void formation, leading eventually, to both electrical and mechanical failure of the interconnect element.
Present means of decreasing electromigration include using barrier metals or dopants in the solder. These means, however, present their own reliability issues and can lead to cost increases that outweigh their effectiveness. Accordingly, further means for reducing electromigration are needed.